Bands who commonly play in:
Drop D: Nickelback, Billy Talent, various hard rock/alternative
Drop Db: Linkin Park, Disturbed, various hard rock/alternative/metal
Drop C: System of a Down, Three Days Grace, various heavy metal
Drop B: Slipknot, various death metal
On the other end of the spectrum, drop d is used acoustically for a lot of blues music.
"Ode" by Creed
"Never Again" by Nickelback
Fantastic Songs in Drop D:
Most Pain of Salvation songs
Opeth - Demon of the Fall, Blackwater Park
Rush - Between the Wheels and Stick it Out
Live - Lakini's Juice
Dropped D tuning is perhaps most widely used in heavy metal or rock music as they take full advantage of the easier power chord transitions needed for extremely fast playing. However, the tuning has also been used in many other types of music, including blues, country, and folk. Most modern metal guitarists use or have used Drop D tuning.
This tuning may be the most prominent tuning in modern rock and metal, but it is not uncommon for these guitarists to play in E-flat tuning/down tuning or dropped C tuning.
Drop D also allows fingerpickers to play chord shapes higher up the neck while maintaining an alternating bass.
List of guitars by dificulty of playing:
1 Reular Guitar
2 Guitar Hero game
3 Elmo's play-along guitar
4 Guitar tuned to drop D
Drop D is generally used by talentless nu-metal bands who cannot actually play anything noteworthy.
Alternate tuning isn't always bad. Real bands like Led Zeppelin use open G and open F for slide guitar and harmonics with advanced picking.